There is a good free RBL that checks users against a database of known forum spammers. You can check a users IP against it before allowing them to do anything on your site. Here is some example PHP code:

If you have some PHP code, and you want to check to see if an IP address is of a known spammer, put the function in the first post of this thread into your PHP script. WHERE exactly I cannot tell you as that is different in every script.

Then you just pass the IP to the function and it will tell you true or false:

Code:

if (blacklisted('123.123.111.123'))
{
## This IP is blacklisted!
}

If you think you can just plug this into any script without any programming knowledge, then I would say no. You have to have some understanding how the code is working. This is a generic function, not a specific one for a particular program.